1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires, particularly a pneumatic radial tire for construction vehicles having a lug pattern, and more particularly to a heavy duty pneumatic radial tire improving a crack resistance at groove bottom in a lug groove for the formation of lug and a resistance to chunk-out from a bottom of a lug groove in a tread rubber while maintaining wear resistance, traction performance, resistance to heat build-up and the like.
2. Description of Related Art
The heavy duty pneumatic radial tires, particularly pneumatic radial tires for construction vehicle are required to ensure traction performance and gripping performance in the running on a non-paved road surface, so that a lug pattern is frequently adopted.
Since a large driving torque is applied to a tread portion of a tire mounted onto a driving shaft of the construction vehicle, there is frequently caused a chance of creating cracks at a groove bottom of a lug groove for the formation of a lug. In order to prevent the occurrence of the crack, there are used a way that a radius of curvature in the groove bottom at a section of the lug groove in a direction perpendicular to the lug groove is made large to disperse strain or stress and the like.
If the crack is created in the groove bottom, there is sometimes caused a failure that the lug is chunked out from the belt (chunking of rubber lump) due to the growth of the crack in the running of the tire. This failure is countered by increasing a gauge of a skid base. And also, such an increase of the gauge has an effect of raising disposal limit of tread rubber due to the wearing.
However, the increase of the radius of curvature in the groove bottom for preventing the occurrence of cracks at the groove bottom of the lug groove is regulated by the width of the lug groove and is naturally critical, so that the satisfactory effect can not be obtained by such a way.
And also, the increase of the gauge in the skid base is an increase of gauge in the tread rubber as a whole, which brings about the increase of heat build-up and rise of temperature in the tread rubber and finally degrades the resistance to heat build-up in the tire. Therefore, such an increase of the gauge is unsuitable for tires used under a high-speed running condition.
It is, therefore, an object of the invention to provide a heavy duty pneumatic radial tire capable of advantageously improving both crack resistance at the bottom of the lug groove and resistance to chunk-out while maintaining traction performance and resistance to heat build-up at conventional levels.
According to the invention, there is the provision of in a heavy duty pneumatic radial tire having a lug pattern formed on both sides of a tread portion, the improvement wherein each of lug grooves forming lugs is extended from a position separated from an equatorial plane of the tire by a width L0 corresponding to 3-15% of a ground contact width up to each of both ends of a tread portion and opened at such a tread end, and the lug groove has a platform on either a continuous region ranging from a top end portion near to the equatorial plane to a central portion in a longitudinal direction of the lug groove and connecting the top end portion to the central portion or a division region separated the top end portion and the central portion from each other, and the platform has such a surface shape that a groove depth at the top end portion is made shallower than a groove depth at the central portion.
The term xe2x80x9cground contact widthxe2x80x9d used herein is according to the definition of xe2x80x9cground contact widthxe2x80x9d described on xe2x80x9cChapter G: General Informationxe2x80x9d of JATMA YEAR BOOK 1999, xe2x80x9cSection 4xe2x80x9d of The Tire and Rim Association, Inc.-1999 and Standard Manual-1999, Earthmoving Equipment Tyres of European Tyre and Rim Technical Organization, provided that xe2x80x9crequired air pressurexe2x80x9d in the same chapter is a maximum air pressure of the respective tire and xe2x80x9crequired massxe2x80x9d is a mass of a maximum load capacity.
In a preferable embodiment of the invention, the platform located on the continuous region connecting the top end portion of the lug groove to the central portion thereof has such a surface shape that the groove depth of the lug groove is gradually increased from a start end of the top end portion toward an upper edge of a terminal end of the central portion.
In another preferable embodiment of the invention, the platform located on the continuous region or the division region at the side of the top end portion is existent in such a place that a ratio of distance L1 on a vertical line drawn from the position of the top end of the lug groove to the tread end to a vertical distance L ranging from the position of the top end of the lug groove to the tread end corresponding to an end of a ground contact width in a ground contact face of the tread portion is within a range of 0.1-0.2 and satisfies such a condition that a ratio of height h1 of the platform from a bottom line of the lug groove to height H1 from the bottom line to the ground contact face on a normal line drawn to the ground contact face is within a range of 0.2-0.6.
In the other preferable embodiment of the invention, the platform located on the continuous region or the division region at the side of the central portion is existent in such a place that a ratio of distance L2 on a vertical line drawn from the position of the top end of the lug groove to the tread end to a vertical distance L ranging from the position of the top end of the lug groove to the tread end corresponding to an end of a ground contact width in a ground contact face of the tread portion is within a range of 0.2-0.5 and satisfies such a condition that a ratio of height h2 of the platform from a bottom line of the lug groove to height H2 from the bottom line to the ground contact face on a normal line drawn to the ground contact face is within a range of 0.1-0.4.